Early work relating to Wireless Location Systems is described in U.S. Pat. No. 5,327,144, Jul. 5, 1994, “Cellular Telephone Location System,” which discloses a system for locating cellular telephones using novel time difference of arrival (TDOA) techniques. Further enhancements of the system disclosed in the '144 patent are disclosed in U.S. Pat. No. 5,608,410, Mar. 4, 1997, “System for Locating a Source of Bursty Transmissions.” Both of these patents are assigned to TruePosition, Inc., the assignee of the present invention, and both are incorporated herein by reference. TruePosition has continued to develop significant enhancements to the original inventive concepts and have developed techniques to further improve the accuracy of Wireless Location Systems while significantly reducing the cost of these systems. Patents relating to such enhancements include, but are not necessarily limited to: U.S. Pat. No. 6,091,362, Jul. 18, 2000, “Bandwidth Synthesis for Wireless Location System”; U.S. Pat. No. 6,097,336, Aug. 1, 2000, “Method for Improving the Accuracy of a Wireless Location System”; U.S. Pat. No. 6,115,599, Sep. 5, 2000, “Directed Retry Method for Use in a Wireless Location System”; U.S. Pat. No. 6,172,644 B1, Jan. 9, 2001, “Emergency Location Method for a Wireless Location System”; and U.S. Pat. No. 6,184,829 B1, Feb. 6, 2001, “Calibration for Wireless Location System.”
Over the past few years, the cellular industry has increased the number of air interface protocols available for use by wireless telephones, increased the number of frequency bands in which wireless or mobile telephones may operate, and has expanded the number of terms that refer or relate to mobile telephones to include “personal communications services”, “wireless”, and others. The air interface protocols now include AMPS, N-AMPS, TDMA, CDMA, GSM, TACS, ESMR, GPRS, EDGE, and others. The changes in terminology and increases in the number of air interfaces do not change the basic principles and inventions discovered and enhanced by the inventors. However, in keeping with the current terminology of the industry, the inventors now call the system described herein a Wireless Location System.
The inventors have conducted extensive experiments with the Wireless Location System technology to demonstrate both the viability and value of the technology. For example, several experiments were conducted during several months of 1995 and 1996 in the cities of Philadelphia and Baltimore to verify the system's ability to mitigate multipath in large urban environments. Then, in 1996 the inventors constructed a system in Houston that was used to test the technology's effectiveness in that area and its ability to interface directly with E9-1-1 systems. Then, in 1997, the system was tested in a 350 square mile area in New Jersey and was used to locate real 9-1-1 calls from real people in trouble. Since that time, the system test has been expanded to include 125 cell sites covering an area of over 2,000 square miles. During all of these tests, techniques discussed and disclosed herein were tested for effectiveness and further developed, and the system has been demonstrated to overcome the limitations of other approaches that have been proposed for locating wireless telephones.
The value and importance of the Wireless Location System has been acknowledged by the wireless communications industry. In June 1996, the Federal Communications Commission issued requirements for the wireless communications industry to deploy location systems for use in locating wireless 9-1-1 callers, with a deadline of October 2001. The location of wireless E9-1-1 callers will save response time, save lives, and save enormous costs because of reduced use of emergency responses resources. In addition, numerous surveys and studies have concluded that various wireless applications, such as location sensitive billing, fleet management, and others, will have great commercial values in the coming years.
Background on Wireless Communications Systems
There are many different types of air interface protocols used for wireless communications systems. These protocols are used in different frequency bands, both in the U.S. and internationally. The frequency band does not impact the Wireless Location System's effectiveness at locating wireless telephones.
All air interface protocols use two types of “channels”. The first type includes control channels that are used for conveying information about the wireless telephone or transmitter, for initiating or terminating calls, or for transferring bursty data. For example, some types of short messaging services transfer data over the control channel. In different air interfaces, control channels are known by different terminology, but the use of the control channels in each air interface is similar. Control channels generally have identifying information about the wireless telephone or transmitter contained in the transmission. Control channels also include various data transfer protocols that are not voice specific—these include General Packet Radio Service (GPRS), Enhanced Data rate for GSM Evolution (EDGE), and Enhanced GPRS (EGPRS).
The second type includes voice channels that are typically used for conveying voice communications over the air interface. These channels are only used after a call has been set up using the control channels. Voice channels will typically use dedicated resources within the wireless communications system whereas control channels will use shared resources. This distinction will generally make the use of control channels for wireless location purposes more cost effective than the use of voice channels, although there are some applications for which regular location on the voice channel is desired. Voice channels generally do not have identifying information about the wireless telephone or transmitter in the transmission. Some of the differences in the air interface protocols are discussed below:
AMPS—This is the original air interface protocol used for cellular communications in the U.S. In the AMPS system, separate dedicated channels are assigned for use by control channels (RCC). According to the TLA/EIA Standard IS-553A, every control channel block must begin at cellular channel 333 or 334, but the block may be of variable length. In the U.S., by convention, the AMPS control channel block is 21 channels wide, but the use of a 26-channel block is also known. A reverse voice channel (RVC) may occupy any channel that is not assigned to a control channel. The control channel modulation is FSK (frequency shift keying), while the voice channels are modulated using FM (frequency modulation).
N-AMPS—This air interface is an expansion of the AMPS air interface protocol, and is defined in EIA/TIA standard IS-88. The control channels are substantially the same as for AMPS; however, the voice channels are different. The voice channels occupy less than 10 KHz of bandwidth, versus the 30 KHz used for AMPS, and the modulation is FM.
TDMA—This interface is also known D-AMPS, and is defined in EIA/TIA standard IS-136. This air interface is characterized by the use of both frequency and time separation. Control channels are known as Digital Control Channels (DCCH) and are transmitted in bursts in timeslots assigned for use by DCCH. Unlike AMPS, DCCH may be assigned anywhere in the frequency band, although there are generally some frequency assignments that are more attractive than others based upon the use of probability blocks. Voice channels are known as Digital Traffic Channels (DTC). DCCH and DTC may occupy the same frequency assignments, but not the same timeslot assignment in a given frequency assignment. DCCH and DTC use the same modulation scheme, known as π/4 DQPSK (differential quadrature phase shift keying). In the cellular band, a carrier may use both the AMPS and TDMA protocols, as long as the frequency assignments for each protocol are kept separated. A carrier may also aggregate digital channels together to support higher speed data transfer protocols such as GPRS and EDGE.
CDMA—This air interface is defined by EL/TIA standard IS-95A. This air interface is characterized by the use of both frequency and code separation. However, because adjacent cell sites may use the same frequency sets, CDMA is also characterized by very careful power control. This careful power control leads to a situation known to those skilled in the art as the near-far problem, which makes wireless location difficult for most approaches to function properly. Control channels are known as Access Channels, and voice channels are known as Traffic Channels. Access and Traffic Channels may share the same frequency band, but are separated by code. Access and Traffic Channels use the same modulation scheme, known as OQPSK. CDMA can support higher speed data transfer protocols by aggregating codes together.
GSM—the international standard Global System for Mobile Communications defines this air interface. Like TDMA, GSM is characterized by the use of both frequency and time separation. The channel bandwidth is 200 KHz, which is wider than the 30 KHz used for TDMA. Control channels are known as Standalone Dedicated Control Channels (SDCCH), and are transmitted in bursts in timeslots assigned for use by SDCCH. SDCCH may be assigned anywhere in the frequency band. Voice channels are known as Traffic Channels (TCH). SDCCH and TCH may occupy the same frequency assignments, but not the same timeslot assignment in a given frequency assignment. SDCCH and TCH use the same modulation scheme, known as GMSK. GSM can also support higher data transfer protocols such as GPRS and EGPRS.
Within this specification the reference to any one of the air interfaces may refer to all of the air interfaces, unless specified otherwise. Additionally, a reference to control channels or voice channels may refer to all types of control or voice channels, whatever the preferred terminology for a particular air interface. Finally, there are many more types of air interfaces used throughout the world, and there is no intent to exclude any air interface from the inventive concepts described within this specification. Indeed, those skilled in the art will recognize other interfaces used elsewhere are derivatives of or similar in class to those described above.